LibGL: Turn Sampler2D into an actual class

This extracts the sampler functionality into its own class.
Beginning with OpenGL 3 samplers are actual objects, separate
from textures. It makes sense to do this already as it also
cleans up code organization quite a bit.

Userland/Libraries/LibGL/CMakeLists.txt

@@ -1,5 +1,6 @@
 set(SOURCES
     Tex/NameAllocator.cpp
+    Tex/Sampler2D.cpp
     Tex/Texture2D.cpp
     Tex/TextureUnit.cpp
     Clipper.cpp